In previous hybrid vehicles, power consumption of an electric motor for driving is generally smaller than that of an engine. Therefore, the amount of heat generated from such as the driving electric motor or a driving electric circuit for driving the driving electric motor, including an inverter circuit, is generally small. Accordingly, a heat radiating capacity required to a cooling system for such vehicles is not so large.
In recent years, however, the hybrid vehicles are required to have a driving performance or an accelerating performance on the same level as that of normal vehicles, which are driven only by engines. With this, power consumption of the driving electric motor tends to increase, resulting in an increase in the amount of heat generation at the electric motor for driving and the driving electric circuit.
To address this issue, it may be conceived to increase a circulation rate of cooling water by enlarging a pump device, which circulates the cooling water having absorbed waste heat generated from an heating element such as the driving electric motor, the driving electric circuit, or the like. However, this results in an increase in power consumption of the pump device. Furthermore, if the pump device is enlarged, it may be difficult to ensure a space for mounting the large pump device in an engine compartment.
It is also conceived to increase a heat radiating capacity of a radiator. However, this results in an increase in a size of the radiator. Thus, it is also difficult to ensure the space for mounting such a large radiator in the engine compartment.